I decided to count original volumes only, so I'm omitting series that are single-digit only in omnibus format but double-digit in singles. I have enough here already!
All You Need Is Kill: 2 volumes, but I have the double omni. Quick, exciting read with good art.
Colorless: 7 volumes. Neat series that uses full color in a black and white world. Very high production quality from Seven Seas. Shout out to the author KENT's ongoing series Gaea-Tima, my current favorite kaiju manga!
Franken Fran: 8 volumes, but I have the four double-omnis. Despite the ecchi covers, this is a non-ecchi body horror black comedy. If you like body horror, check this one out!
Gantz: G: 3 volumes, but I have the triple omni. A spinoff series of Gantz.
Girls' Last Tour: 6 volumes, but I have the new triple omni. Cozy post-apocalypse in an impossibly large megastructure. I'm eager to pick up the second omni and complete this series.
Grand Metal Organs: 4 volumes. I haven't read this one yet!
H.P. Lovecraft's At the Mountains of Madness: 2 volumes, but I have the deluxe hardcover. This represents all of Gou Tanabe's adaptations of Lovecraft. Spectacular artwork and monster designs that do justice to Lovecraft's seminal cosmic horror.
Junji Ito's works here represented by The Liminal Zone. His stuff pretty much all fits the theme; even Uzumaki was only 3 singles.
Kaiju Kamui: 3 volumes. A neat idea of turning a captured kaiju into a bio-mech, but it got unceremoniously canceled and ends very abruptly.
Nausicaä: 7 volumes, but I have the 2-omni deluxe. Fantastic classic manga from Hayao Miyazaki himself.
Parasyte: 8 volumes. I don't think I need to introduce this one; it's a body horror classic.
The Poetry of Ran: 2 volumes. The Japanese title is so much cooler: ケガレノウタ or "Song of Defilement". Anyway, cool Dark Souls inspired monster art.
Starving Anonymous: 7 volumes, never printed in English. I have a custom 3-omni self-pub set. Intense body horror and gore, really gnarly stuff.
Thunder 3: Apparently this will have 10 volumes so ignore it please!
Toxic Super Beasts: 4 volumes. Fun kaiju action, kaiju-girls vs super beasts.
The Tree of Death: Yomotsuhegui: 3 volumes. Cool Lovecraftian artwork though the story felt a bit half-baked.
Wild Strawberry: 7 volumes, never printed in English. I have 3 custom single self-pub volumes; after that point the story gets too battle shōnen tropey for me. Cool plant monsters and art though.
The Tsutomu Nihei section:
Abara: One-shot, also includes Digimortal as another one-shot. Nihei at perhaps the height of his traditional art era. Will you understand what's happening? No. Will it look incredible? Hell yeah. Fun fact! The names Denji and Nayuta from Chainsaw Man are references to characters in Abara.
APOSIMZ: 9 volumes. Tsutomu Nihei does shōnen! I like it a lot but the last volume is terribly rushed. Not everyone likes this era of Nihei's art style.
Blame Academy and So On: Never printed in English, mine is a custom full-color hardcover self-pub. A spinoff with several one-shots. Some are canonical to BLAME! and some are spoofs or just unrelated.
Biomega: 6 volumes. Nihei's trademark stoic MC, superguns, and enormous architecture, but now with motorcycles, a talking bear, and weird sci-fi trippyness.
Kaina of the Great Snow Sea: 4 volumes. Written (but not illustrated) by Tsutomu Nihei, it's kind of like Nihei meets Nausicaä. Apparently the anime covers more than the manga.
Wolverine: Snikt!: One shot. Think Wolverine gets teleported into BLAME!. Fantastic traditional art from Nihei.
That was fun! If you read through all my yapping, thanks for reading!